Mounting device for oscillatory crystal which converts torsional vibrations to flexural vibrations

ABSTRACT

Wire suspension device for piezoelectric oscillatory crystals in which the wires are fixed on the faces thereof by a part of their lateral surface acting as a conversion lever, replacing the usual wire&#39;&#39;s &#39;&#39;&#39;&#39; lambda /4 &#39;&#39;&#39;&#39; torsional mechanical oscillations by flexional oscillations, thus enabling use of short wires and improving aging stability.

United States Patent [1 1 Gibert et al.

[4 1 Sept. 16, 1975 4] MOUNTING DEVICE FOR OSCILLATORY CRYSTAL WHICHCONVERTS TORSIONAL VIBRATIONS TO FLEXURAL VIBRATIONS [76] Inventors: GuyGibert; Pierre Vidal, both of 101, rue du President Roosevelt,78-Sartrouville, France [22] Filed: May 18, 1973 [2]] Appl. No.: 361,513

Related US. Application Data '[63] Continuation-impart of Ser. No.223,604, Feb. 4,

[30] Foreign Application Priority Data Feb. 26, 1971 France 71.06695 52U.S.Cl ..310/9.1;31o/s.2 s11 1nt.Cl. ..H0ll4l/04 5s FieldofSearch...31o/s.2,9.1,9.4,9.7,

[56] References Cited UNITED STATES PATENTS 2,410,825 11/1946 Lane"...-3lO/9.8 X 3,566,164 2/1971 Boillat.... 3lO/9.1 3,581,126 5/1971 Omlin310/9.l 3,751,692 8/1973 Choffat 3lO/9.1

Primary Examiner-Mark O. Budd Attorney, Agent, or Firm-Cushman, Darby &Cushman ABSTRACT Wire suspension device for piezoelectric oscillatorycrystals in which the wires are fixed on the faces thereof by a part oftheir lateral surface acting as a conversion lever, replacing the usualwires M4 torsional mechanical oscillations by flexional oscillations,thus enabling use of short wires and improving aging stability.

6 Claims, 5 Drawing Figures PATENTED SEP I 6 I975 SHEET 1 UP 2 MOUNTINGDEVICE FOR OSCILLATORY CRYSTAL WHICH CONVERTS TORSIONAL VIBRATIONS TOFLEXURAL VIBRATIONS This application is a continuationin-part of mycopending application Ser. No. 223.604 filed Feb. 4, I972.

The present invention relates to oscillatory crystals for time-basepurposes, these conventionally being electrically sustained by thepiezoelectric effect through suitably disposed electrodes, and thematerial of which they are made being usually constituted by a quartzcrystal cut in particular directions which are associated with itscrystal lattice.

In the case of the low frequencies, ranging for example from a fewkilohertz up to a few tens of megahertz, the crystal elements used takethe form of elongated bars whose section is near-square, and which areequipped with electrodes whose shape and disposition are such that theresultant vibrations involve a bending or flexion motion of the wholebar.

However, the need to attach the vibrating crystal in mountings, normallyby soldering operations, creates a substantial difficulty, namely thatof the transmission of mechanical energy from the crystal to itsmountings or supporting arrangements. This in turn means thatmodifications to the operating characteristics of the crystal on itsown, are introduced; moreover, said modifications develop over a periodof time, as a consequence of the aging of the soldered joints referredto.

These phenomena can be reduced by selecting upon the crystals, as pointsof attachment for the suspension elements, points which are usuallyreferred to as nodal points where the amplitude of the vibration isminimum.

However, due to the development of industrial requirements, crystals ofdecreasing size have to be used, in relation to which, the dimensions ofthe suspension element have ceased to be negligible. This has given riseto the increasing problem for providing accurate and perfectlyreproducible crystal fixings, with the consequence that ultimately theregularity of manufacture and time-based stability, have becomemediocre.

The present invention relates to a suspension device which does notexhibit this drawback and more particularly to a fixing device for acrystalline bar of parallelepiped form which is capable of mechanicaloscillations in a flexion mode and in a plane parallel to that of thefixing faces of said parallelepiped, said bar comprising a fixedsupport, and suspension wires respectively attached by one of the endsthereof to said support and at their other end to the crystal, at pointsof minimum vibration on the fixing faces thereof, said other end of eachof said wires being fixed to said faces by a part of the lateral surfaceof the wire.

The invention will be better understood from a consideration of theensuing description and the attached figures in which:

FIG. 1 illustrates a suspension device of prior art design;

FIG. 2 illustrates a suspension device in accordance with the invention;

FIGS. 3, 4 and 5 illustrates various embodiments of the invention.

FIG. 1 illustrates a suspension device in accordance with the prior art,applied to a crystal bar operating in the flexion mode. The devicecomprises a fixed support 1 to which there are attached suspension wires2 whose ends terminate, at the surface of the crystal bar 3,at nodalpoints, or points of minimum vibration as defined hereinbefore, namely4, 5, 6 and 7.

These points are located upon the longitudinal axes of the faces 8 and 9at special distances from the ends, which distances, taking the flexionmode of vibration employed here, are equal to 0.224 times the overalllength of the bar.

In operation, under the influence of electric fields applied to thepiezoelectric bar electrodes, which have not been shown, the flexionmode oscillation takes place, and causes an alternating deformation ofthe neutral axis between the two extreme positions 13 and 14 illustratedin FIG. 1, which are exaggerated in amplitude, for the sake of clarity.

This alternating movement induces an alternating torsional stress in thesuspension wire,by virtue of the rotation of its extremity in thedirection of the arrows- ,at the frequency of vibration of the crystal.these torsional oscillations being propagated up, along the wire,to thatpart of the fixed support to which the suspension is attached. Thesuspension wire thus behaves as a transmission line carrying alternatingmechanical energy, and the decisive role which this phenomenon plays onthe correct operation of the crystal has been referred to hereinbefore.

It is well known, in the field of this kind of transmission linearrangement, to reduce the transmitted energy by causing thetransmission line to vibrate in the socalled quarter-wave" mode, thisleading to a special value for the length of the suspension wires whichis given by the relationship:

where d is the free length of the suspension wire;

fthe frequency of oscillation of the crystal;

V is the velocity propagation of the oscillation along the wire; and

k is the wavelength of propagation of the oscillation.

For reasons of size, it is desirable that the length d of the suspensionwire should be short and this means that, for a given frequency fdetermined by each particular application, a velocity of propagation Vhas to be sought which is likewise low. However, the torsional mode ofvibration described hereinbefore for a suspension of the prior art, asillustrated in FIG. I, corresponds to the contrary to a high velocitypropagation through the wire, and does not enable efficient control ofthe reflection conditions at its ends to be achieved.

FIG. 2 illustrates a first embodiment of the device, in accordance withthe invention where, for the sake of clarity, and this applying to thefollowing figures as well, only the top part of the device has beenshown.

The suspension wire is attached to the crystal by the provision of afold 12 of its end, defining a segment 30 parallel to the fixing face,the one extremity 31 of this segment being fixed at points of minimumvibration of the fixing face.

In operation, this segment 30, thus constituted by a part of the wireitself, acts as an oscillating lever, convetting the conventionaltorsional oscillation of the wire into a flexion oscillation, the wireconsidered about its mean position 2, occupying extreme positions 19 and20; this flexion mode corresponds to a very much lower velocitypropagation, this being a factor the need for and advantages of whichhave been discussed hereinbefore.

It is to be noted that, for achieving the conversion operation of thelever, only one of its extremities 31 must be fixed on the crystal,usually by solder, excluding particularly the folded portion, or elbowportion 12 at the other lever extremity thereby securing a free movementfor the oscillating lever segment.

Thus, a characteristic of the invention is that the lateral part of thewire acting as a conversion lever must be freeof any fixating oranchoring material such as a solder, except at its very extremity 31which is fixed to the crystal. As an order of magnitude in practicalcases, the free portion of the lever must have a length greater than thetwo diameters of the wire suspension.

FIG. 3 illustrates a variant embodiment of the invention which lies on ageometric characteristic of the oscillating of the crystal, namely, theexistence of nodal verter lever. The operation is the same as that ofthe device shown in FIG. 2, but the amplitudes of oscillation arereduced, and consequently the crystal has improved mechanical isolation.

FIG. 4 illustrates another simplified variant embodiment of theinvention,'in which the free part of the suspension is contained in aplane parallel to that of the face of the crystal to which it isattached by the lever 24. The operation is the same as that of thedevices of FIGS. 2 and 3 although there is an improvement in theconversion of one mode of oscillation to the other, because of thesimplification of the mechanical structure, albeit at the expense of agreater longitudinal dimension.

FIG. 5 illustrates a variant embodiment of the device shown in FIG. 4,in which the free part of the suspension has been given a rectangularsection, the shorter side being attached to the crystal and the longerside being perpendicular to the fixing face of the crystal.

This kind of section, although enabling the suspension to oscillate inthe plane parallel to the fixing face of the crystal, prevents it fromdoing so in the perpendicular plane and thus strictly defines theeffective plane of vibration.

By way of example, the invention has been applied to crystal oscillatorsoperating at low frequencies in the order of a few kilohertz, and hasshown a major improvement in the quality factor of the crystal, e.g., onthe order of a ratio of 2, and, compared with the case of crystalssuspended by the conventional method, a substantial reduction in thetime-based frequency drift normally brought about due to slowmodifications in the mode of suspension.

Of course, the invention is not limited to the embodiment described andshown which was given solely by way of example.

What is claimed is:

1. In a fixing device for an oscillatory crystal bar of parallelepipedform capable of mechanical oscillations in a flexion mode in a planeparallel to that of the fixing faces of said parallelepiped, mountingmeans for converting torsional vibrations to flexional vibrations, saiddevice comprising a fixed support, and a plurality of suspension wireseach being attached at one end thereof to said support, and at the otherend thereof being formed into a lever aligned parallel to sid flexionplane, the extremity of said lever portion of each of said wires beingfixed on its lateral surface to the fixing faces at points of minimumvibration wherein the portion of each of said suspension wires betweensaid one end and said other end having a length equal to a quarter ofthe wavelength of propagation of flexional oscillations in saidsuspension wires, the frequency of said flexion oscillations being equalto the frequency of said mechanical oscillations of said crystal bar.

2. A fixing device as claimed in claim 1, wherein said lever portion ofeach of said wires makes an angle with the remaining portion of saidwire attached to said fixed support.

3. A fixing device as claimed in claim 2, wherein said angle is a angle.

4. A fixing device as claimed in claim 3, wherein said remainingportions of said wires attached to said support each makes a 90 anglewith said fixing faces.

5. A fixing device as claimed in claim 2, wherein the portion of eachwire attached to said support is parallel to said fixing faces.

6. A fixing device as claimed in claim 5, wherein said lever portion hasa rectangular transverse section and wherein the short side of saidrectangular section is attached to said fixing faces.

l l l

1. In a fixing device for an oscillatory crystal bar of parallelepipedform capable of mechanical oscillations in a flexion mode in a planeparallel to that of the fixing faces of said parallelepiped, mountingmeans for converting torsional vibrations to flexional vibrations, saiddevice comprising a fixed support, and a plurality of suspension wireseach being attached at one end thereof to said support, and at the otherend thereof being formed into a lever aligned parallel to sid flexionplane, the extremity of said lever portion of each of said wires beingfixed on its lateral surface to the fixing faces at points of minimumvibration wherein the portion of each of said suspension wires betweensaid one end and said other end having a length equal to a quarter ofthe wavelength of propagation of flexional oscillations in saidsuspension wires, the frequency of said flexion oscillations being equalto the frequency of said mechanical oscillations of said crystal bar. 2.A fixing device as claimed in claim 1, wherein said lever portion ofeach of said wires makes an angle with the remaining portion of saidwire attached to said fixed support.
 3. A fixing device as claimed inclaim 2, wherein said angle is a 90*angle.
 4. A fixing device as claimedin claim 3, wherein said remaiNing portions of said wires attached tosaid support each makes a 90* angle with said fixing faces.
 5. A fixingdevice as claimed in claim 2, wherein the portion of each wire attachedto said support is parallel to said fixing faces.
 6. A fixing device asclaimed in claim 5, wherein said lever portion has a rectangulartransverse section and wherein the short side of said rectangularsection is attached to said fixing faces.